The field of this invention is wear and fatigue resistant clutches for screw guns which automatically disengage when a screw is driven to a selected depth and which disengage positively to prevent clashing, and also materials and processes for manufacturing wear and fatigue resistant alloy steel components for screw gun clutches, and for power tools generally, especially hand-held power tools.
The prior art includes several examples of positive disengagement clutches, or “quiet” clutches, for screw guns. Generally, these clutches include a mechanism for moving the clutch surfaces away from one another when they disengage to prevent the clutch surfaces from clashing. This positive disengagement leads to an extended life of the clutch and results in other desirable operating characteristics such as reduced noise and vibration.
U.S. Pat. Nos. 4,655,103, 4,809,572, and 4,947,714 disclose exemplary quiet clutches for screw guns. The '714 patent discloses a clutch with three clutching elements—a drive element, an intermediate element and an output element. To drive a screw, all three clutch elements are initially engaged with one another (see FIG. 2 of the '714 patent). At some point while driving the screw, torque causes the intermediate element and the output element to separate from each other due to the cam surfaces 30, 15 (see FIG. 3 of the '714 patent). When the screw is almost driven to its intended depth, the intermediate element and the output element slide axially forward away from the input element, following the screw into the work piece (see FIG. 4 of the '714 patent). When the screw reaches its intended depth, the input element and the intermediate element slip. Once the slip occurs, the torque is released and the separation between the output element and the intermediate element caused by the torque and the cam surfaces 30 and 15 is no longer present. A spring positioned between the input element and the intermediate element returns the intermediate element back together with the output element. This return creates a gap “s” between the input element and the intermediate element (see FIG. 5 of the '714 patent). The gap prevents clashing of the clutch surfaces during disengagement.
While the clutch described in the '714 patent prevents clashing of the clutch surfaces during disengagement, the contact area between the intermediate element and the output element in the '714 patent decreases as the intermediate element moves away from the output element. This decrease in the contact area leads to an additional increase in the stress which arises on the intermediate element and the output element from the transfer of torque from one to the other. The increased stress could lead to a decrease in the fatigue life of each part.
Clutch components such as the drive element, intermediate element and output element described in the '714 patent have been made from steel such as SAE 8620 steel. While this steel has been selected because of its desirable wear and fatigue properties, clutches in screw guns nevertheless remain one of the shortest-lived wear components in these tools. Breakdown of screw guns due to clutch failure causes the user to spend considerable time and money repairing the clutch or replacing the tool in its entirety.
Screw gun clutches, as well as other high-wear components in power tools, would benefit from greater wear and fatigue resistance properties. Many high-wear components in hand-held power tools are especially sensitive to wear and fatigue because in hand-held power tools the weight and size of these components is typically minimized. Use of higher alloy steels can improve the wear and fatigue resistance of these components but does so typically at the expense of the cost competitiveness of the tool. Higher alloy steels can lead to greater initial manufacturing and inventory costs and increased difficulty in final machining.
The wear and fatigue resistance of screw gun clutches is improved herein through both improvements in the design of these clutches and improvements in the materials and manufacturing methods for producing them. These improvements are achieved with very little increase in the cost of the clutches.
Thus, one aspect of the invention is a clutch design which prevents clashing of the clutch surfaces during disengagement and is also more wear resistant than previous designs. This clutch design is also simple and inexpensive.
Another aspect of the invention is materials and methods for manufacturing the components of the clutch. The components can be cast to near-net shape with a higher alloy steel, and can later be carburized or nitrided and subjected to other treating processes. In addition to screw gun clutches, these materials and methods are also applicable to other high-wear components in power tools generally, such as clutch components for drill/drivers and rotary ratchets for hammerdrills and hammers.